Tree seedling recruitment, encompassing germination and growth, is crucial for natural and artificial propagation. Recruitment depends on the interplay of biotic and abiotic modifiers and can be influenced by experimental treatments and nursery methodologies, such as seed priming techniques and seedling treatments, which affect the success of forest restoration and nursery operations. Despite extensive research, the effects of natural modifiers and treatments across species remain poorly understood. We conducted a systematic review of 91 peer-reviewed articles to identify patterns and knowledge gaps on the effects of natural modifiers, seed priming, and seedling treatments on seedling recruitment. Our Scopus search focused on experimental studies of tree germination and seedling fitness in temperate and subtropical species. Starting with 266 articles, we critically appraised them for internal validity using two sets of criteria and independent evaluators. Data on tree responses of studies with internal validity were categorized and analyzed narratively. Most reviewed studies (60.5%) focused on abiotic modifiers. Seedling-focused studies (66.6%) outnumbered seed-focused ones (33.4%). Overall, 42.2% of the studies reported nonsignificant effects. Fertilization, temperature, precipitation, pollution, and light availability had mostly nonsignificant effects on seedling recruitment. Soil moisture, snow cover, and soil scarification had positive effects, while shading negatively impacted seedlings. Mixed biotic and abiotic modifiers showed inconclusive results. Mycorrhizal associations promoted recruitment, while competition had mixed effects. Seed stratification was more effective than scarification or chemical treatments in promoting germination. To advance understanding of tree recruitment processes, we recommend studies on seedling treatments, mycorrhizal associations, fertilization, warm stratification, scarification, and chemical treatment across species. Frequent nonsignificant results suggest reassessing ecological hypotheses, incorporating context-specific variables, and adopting more robust experimental designs. A trait-based approach seems essential for identifying broader patterns and enhancing conservation and restoration outcomes.